This invention relates to pressure sensing instrumentation, and more particularly to capacitive pressure sensing devices.
There are many forms of pressure sensing instrumentation in the prior art which utilize capacitive sensors. Typically, the sensor in such instrumentation includes a metal housing with metallic diaphragms. The accuracy of these types of instruments is limited by the sensor material, particularly since metals are far from a perfect elastic material, and the characteristic time and stress-dependent memory causes hysteresis and slow drift. Further, metal has relatively high thermal expansion coefficient which in turn causes the instrument to be effected by temperature variation.
Due to such shortcomings in metallic sensors, sensors have been constructed using dielectric capsules with conductive films on the interior surfaces. A typically used dielectric material is quartz, which is characterized by a very low thermal expansion coefficient and near perfect elastic property. Although in theory, quartz could be used to make an ideal pressure sensor, there are many practical considerations which have prevented extensive use of such sensors, notably, high fabrication cost. Typically, prior art dielectric sensors utilize a pair of pre-shaped, cup-like dielectric elements which are joined together, using a suitable sealing material, to form a gas-tight capsule. In other forms, a pair of disc-like dielectric elements are joined together at their peripheral boundary by a bead of sealing material. The sealing material usually is selected to match the thermal-expansion coefficient of the electrical feedthrough material. This material is some form of metal alloy and generally has much higher thermal-expansion coefficient and worse elastic properties than that of the dielectric material. The resultant capsule is therefore made of a composite of materials which are generally inferior than a more homogeneous construction from the thermal stability viewpoint. To reduce the influence of the sealing material, preshaped dielectric elements have been used, although at a greatly increased fabrication cost. Other prior art shows monolithic fused dielectric capsules with metal wire feed-through to make contact with metal film inside the capsule. Since no known metal can match the low coefficient of expansion of otherwise suitable dielectric materials such as quartz, this latter type of construction is impractical due to the non-reliability of the metal-dielectric seal. Furthermore, even satisfactory metal-quartz seals are extremely susceptible to leak or degradation upon subjection to a high temperature (above 2000.degree. F) such as may be required in an annealing process used to reduce internal stress build-up in the fusion process.
Prior art sensors formed from a pair of disc-like dielectric elements joined together at their peripheral boundary by a head of sealing material have been configured as differential pressure sensors by configuring a tubular pressure coupling member through the sealing material so that a reference pressure may be coupled to the capsule interior. However, such configurations have been limited to use in a very narrow temperature range since the sealing material used must accommodate expansion and contraction of both the dielectric elements and the pressure coupling member, while affording a gas-tight seal over a desired operational temperature range.
Even though dielectric materials such as quartz are characterized by a relatively low coefficient of thermal expansion, in certain applications where extremely accurate sensors are required, the temperature dependencies of prior art dielectric capsule sensors cause unacceptably high measurement error. Two principal sources of this error are:
(1) the relatively high thermal expansion rate of the conductive film, and PA1 (2) the thermal coefficient of the modulus of elasticity of the dielectric material.
A further disadvantage of the prior art sensors is their limitation of use to non-corrosive or electrically non-conductive media in order that the media not interact with the electrical connections to the sensors.
Accordingly, it is an object of the present invention to provide a high accuracy capacitive pressure sensing device characterized by a relatively high reliability and low cost.
Another object is to provide a high accuracy capacitive pressure sensing device which is substantially independent of temperature over a relatively broad range of operating temperature.
Still another object is to provide a high accuracy pressure sensing device which is suitable for use in a corrosive or electrically conductive media.